Structural devices are often formed as assemblies of a number of smaller structural members. Such assembling of individual members may be necessary to form devices that are too large or too complicated to be formed by conventional manufacturing methods. For example, such factors as casting sizes, forging sizes, available plate and block sizes, and the like can limit the size and geometry of the individual structural members that can be manufactured. To form larger or more complex devices, the structural members are typically assembled by joining the individual structural members using a variety of known joining techniques including, for example, mechanical fastening or welding.
Joints formed by mechanical fasteners such as rivets, screws, and bolts typically require an overlap of the structural materials at the joint. The fasteners and the overlap of material result in an increase in weight of the joint and the structural assembly. The joint can also introduce areas of increased stress, for example, around holes drilled for receiving rivets. Alternatively, weld joints can be formed to join the structural members, sometimes requiring little or no overlap of material. However, the formation of conventional weld joints, such as by arc or electron beam welding, can result in undesirable dimensional changes in the structural members. Welding can also introduce porosity or other discontinuities into the structural members or otherwise cause unwanted changes to the material properties of the structural members.
Friction welding has also been proposed as an alternative to conventional welding methods for joining members. Weld joints formed by friction welding generally exhibit refined grain structure as compared to weld joints formed by other conventional weld techniques such as plasma arc welding. Linear friction welding and rotational friction welding can be used to form strong joints without reducing the mechanical characteristics of the joined materials or causing significant dimensional changes. However, each of these conventional friction welding techniques is limited by the dimensions of the structural members and/or the joints to be formed. For example, conventional linear friction welding and rotational friction welding require one member to be moved, i.e., oscillated or rotated, and urged against the other member. Because of the difficulty of moving large structural members, it can be impossible or impractical to join some structural members by these techniques.
Thus, there exists a need for an improved apparatus and method of joining structural members to form structural assemblies. Preferably, the method should enable the manufacture of preforms that approximate the desired dimensions and configuration of the structural assembly and therefore require little machining or other subsequent processing to form the structural assemblies. The method should be adaptable for joining large and/or complex structural members. Further, the method should not add significant weight to the structural assembly, and should minimize dimensional changes and undesirable changes to the material properties of the structural members.